The effects of superhelical turns on the structure, fluctuations, and dynamics of closed circular DNAs are studied using two instrumentally sophisticated techniques. The dynamics of collective torsional motions of the filament, which are quite sensitive to occasional interruptions or local weaknesses in its torsional rigidity, are studied using the decay of the fluorescence polarization anisotropy of bound ethidium dye. The dynamics of local motions at the atomic scale are studied by combining such information with nuclear magnetic resonance (nmr) relaxation measurements. These nmr measurements may also directly provide detailed information concerning changes in secondary structure, possibly to the new Z-helical form, induced by the supercoiling.